1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and more particularly, to a PCB with reduced likelihood of a bonding failure due to warpage caused by a surface mounting technology (SMT) process to mount electronic components.
2. Description of the Related Art
As flat panel displays such as organic electroluminescent display devices become thinner and more integrated, liquid crystal display devices are structured to make use of a flexible PCB (FPCB) or rigid-flexible PCB mounted with a plurality of integrated circuits (ICs) to provide connection to a liquid crystal panel. These PCBs use a wire bonding or tape automated bonding (TAB) process to connect components of the flat panel displays.
FIG. 1A is a perspective view illustrating the front surface of a conventional driving printed circuit board provided in a liquid crystal display device. The driving printed circuit board 1 may include a base portion 10, a plurality of control elements 15 bonded to the front surface of the base portion 10, and a flexible portion 17 connecting the base portion 10 to a liquid crystal panel.
Various control elements 15 for controlling a liquid crystal panel and external elements constituting a liquid crystal display device are mounted through a surface mounting technology (SMT) process onto the front surface of the base portion 10. The base portion 10 is made of a single or plural layer structure, and a plurality of pads bonded to the mounted control elements 15 and a plurality of wirings electrically connecting between the pads are formed thereon.
At least one of the control elements 15 may be bonded to a front surface of the base portion 10 through an SMT process, and electrically connected to one another via pads and wirings formed on the base portion 10, and also connected to a liquid crystal panel and other external elements through the flexible portion 17.
The flexible portion 17 can be easily bent without cracking or rupturing to electrically connect the driving printed circuit board 1. The flexible portion 17 may be bent during the module assembly process of a liquid crystal display device and the base portion 10 may be disposed at a rear surface thereof.
FIG. 1B is a perspective view illustrating the rear surface of a conventional driving printed circuit board. A rear surface stiffener plate 20 may be attached to a rear surface thereof to maintain the rigidity of the base portion 10.
The rear surface stiffener plate 20 may be attached to prevent pads and wirings from being disconnected due to the damage of the bonding portion caused when the base portion 10 mounted with the control elements 15 is bent. A polyimide film may be typically used as the rear surface stiffener plate 20. Commercialized polyimide film includes, for example, KAPTON (of DuPont Co.).
However, the method of performing an SMT process using a driving printed circuit board attached to the rear surface stiffener plate 20 may have the following problems.
FIGS. 2A and 2B are views illustrating the front surface of a conventional driving printed circuit board 2 and the rear surface of the same driving printed circuit board 2 after undergoing a conventional SMT process. As illustrated in FIGS. 2A and 2B, the driving printed circuit board 2 is flexible and has a thermal expansion coefficient different from that of the rear surface stiffener plate 21 attached to the driving printed circuit board. The difference in the thermal expansion coefficient causes the driving printed circuit board 2 to be warped as shown in FIGS. 2A and 2B under the pressurization and heating step of the SMT process.
In other words, the degree of contraction in a polyimide film (forming the rear surface stiffener plate 21) is larger than the degree of contraction in the base portion 11 during the cooling stage after the SMT process. The different degrees of contraction cause the base portion 11 to be bent in a rear surface direction after the SMT process. As a result, the mounted control elements 15 may be distorted, or wiring patterns in the printed circuit board 2 may become disconnected.
Such PCB warpage of the printed circuit board may deteriorate the reliability of a driving printed circuit board. Moreover, an additional work may be required to flatten the board after undergoing an SMT process, which causes increase in the time for fabricating the printed circuit board.